How can I change color/oppacity in the menu?

Hi! Newbie here. I don't know much of code or websites, but have been asked to create a website for my company here on squarespace. The problem is that I want to do some things but have no idea how to do them. So, basically, I have in my menu a page that is actually a folder named "Projects". I created two pages in that folder for on going projects and closed projects. The problem is that when I pass my mouse in the menu the background is completely solid, besides the huge size of the titles. It looks weird. I'd like to make it more transparent and reduce the size of the titles, just those 2 titles. How can I do it?

P.S.: The website is in portuguese.

Can you share the website URL? Thanks!

https://mackerel-flounder-c42p.squarespace.com

It hasn't been published yet, but check if you can see it or not. 

I can't, can you add a site wide password?

https://support.squarespace.com/hc/en-us/articles/205815528-Site-wide-passwords

Sure, please check if it's working now. The site-wide password is: proactingtest

Try this Custom CSS:

.header-nav-folder-content {
  background-color:rgba(255,255,255,0.25);
}
.header-nav-folder-item a {
  font-size:1rem;
}

Perfect for the title size! Thank you very much! But the opacity is not working, it's still a solid white. 😕

Should I change something?

It worked! Thank you very much!! Have a great day